WASHINGTON - President Donald Trump's search for a Supreme Court nominee reached the interview stage Monday as he announced that he has spoken to at least four contenders, while remaining coy about his shortlist for a successor to retiring Justice Anthony Kennedy.

"I had a very, very interesting morning," Trump told reporters in the Oval Office as he greeted visiting Dutch Prime Minister Mark Rutte. He described the candidates as "outstanding people" but did not name them, and added that he expects to meet soon with two or three more prospects.

Trump's remarks - coming one week before his own July 9 deadline for a decision - stirred an already intense political standoff over the future of the high court, with the White House setting up a war room Monday to focus on the confirmation process and top Democrats attacking potential nominees.

Trump met Monday with four federal appeals court judges: Brett Kavanaugh, Amy Coney Barrett, Amul Thapar and Raymond Kethledge, according to three people briefed on the meetings who spoke on the condition of anonymity because they were not authorized to speak publicly.

Another judge on Trump's shortlist is federal appeals court Judge Thomas Hardiman, according to White House aides and senior Republicans.

Kavanaugh and Coney Barrett were cited by many Republicans as leading candidates for the court, but officials cautioned that the search remains fluid and that Trump is undecided. Kavanaugh, 53, a veteran of George W. Bush's White House and a former Kennedy clerk, is a favorite of veteran GOP leaders, while Coney Barrett, 46, is being touted by conservatives who came to admire the former Notre Dame Law School professor during her acrimonious confirmation battle last year, when her Catholic faith became a flash point.

"Selecting a conservative female who's intellectually qualified would maximize the president's advantages in trying to get the votes he needs in the Senate and send a good signal to the country," former House Speaker Newt Gingrich said, adding that "the human-interest side of things wouldn't hurt."

On Capitol Hill, Republicans, who hold a narrow 51-seat majority in the Senate, braced for challenges from Democrats and within their ranks, regardless of whom Trump taps.

"I think there will be a big national, campaign rage. But in the end, I'm confident we'll get the judge confirmed," Senate Majority Leader Mitch McConnell, R-Ky., said at an event in Ashland, Kentucky.

But Sen. Rand Paul, R-Ky., a Trump ally, made clear to associates that he has reservations about Kavanaugh, whose writing on President Barack Obama's health-care law has bothered some conservatives. And Sen. Susan Collins, R-Maine, a moderate, expressed concern that Trump's list of candidates includes a "couple of people" who have "demonstrated hostility and an eagerness to overturn Roe v. Wade," the 1973 Supreme Court ruling upholding abortion rights.

"Those individuals would not bring me the kind of assurance I would be seeking," Collins said in an interview with the New York Times.

White House press secretary Sarah Huckabee Sanders told reporters that Trump's meetings with the four candidates Monday were each about 45 minutes. The interviews came on Trump's first day back in Washington after spending the weekend at his golf club in New Jersey.

Sanders insisted that the president did not discuss past cases, such as Roe v. Wade, during the interviews.

"As the president said last week, he's not going to talk to judges about specific cases. He's looking for individuals that have the right intellect, the right temperament and that will uphold the Constitution," she said.

During the 2016 presidential campaign, Trump said willingness to overturn the decision legalizing abortion nationwide would be a litmus test for his court picks.

Democrats, looking to energize their base ahead of the midterms, have begun taking aim at members of Trump's shortlist and see the Supreme Court opening as a chance to rally their voters on social issues, in addition to economic and labor issues.

Senate Minority Leader Chuck Schumer, D-N.Y., on Monday criticized the record of Coney Barrett. On Twitter, Schumer argued that Coney Barrett would support overturning Roe v. Wade. He also pointed to her past statements on the Affordable Care Act and contraception.

"The bottom line: Judge Barrett has given every indication that she will be an activist judge on the Court," Schumer wrote. "If chosen as the nominee, she will be the deciding vote to overturn Roe v. Wade and to strike down pre-existing conditions protections in the ACA."

White House counsel Donald McGahn, a low-key but staunchly conservative figure in the West Wing, is overseeing the confirmation process inside the White House. McGahn worked closely with Trump ahead of Monday's interviews, both conferring with the candidates and briefing the president, according to two White House officials who were not authorized to speak publicly.

McGahn carries significant political capital with the president because of his work on Justice Neil Gorsuch's successful confirmation last year and his close relationship with McConnell and leaders in the conservative legal community, such as Leonard Leo, who is on leave from the Federalist Society and assisting McGahn, the officials said.

Trump's meeting Monday with Thapar, who lives and works in McConnell's home state of Kentucky, was described by several White House aides as both a gesture of respect for the Senate GOP leader and evidence that he is in serious contention.

As part of the search process, McGahn has been placing phone calls in recent days with possible nominees and key senators, including Sen. Mike Lee, R-Utah, who is in both categories. McGahn and Lee spoke by phone Thursday and talked about the confirmation process, according to three people familiar with the conversation who were not authorized to speak publicly. The White House and Lee's office declined to comment.

Paul did not publicly criticize Kavanaugh on Monday but confided to advisers and friends that he has concerns about his judicial record, pointing to his decisions on health care and his work in the Bush White House. Paul's spokesman was unavailable for comment. A White House aide said they were aware of his views but maintained that Paul and others are in "wait-and-see mode."

Other Kavanaugh critics were busy looking at his past statements and building opposition-research files for possible hearings, which both parties expect to be contentious, just months before this year's midterm elections.

One person involved in researching Kavanaugh pointed The Washington Post to a 2002 book by now-liberal activist David Brock, "Blinded by the Right." Brock alleged in the book that at a State of the Union viewing party at conservative commentator Laura Ingraham's house in 1997, Kavanaugh mouthed an expletive about then-first lady Hillary Clinton. The person said that the reported exchange, even if denied at a confirmation hearing, is the kind of anecdote that researchers are pulling up this week.

With a showdown looming, the White House also announced Monday that spokesman Raj Shah is taking a leave of absence from his position to work full time on overseeing the communications effort associated with the Supreme Court pick, part of a broader push by the White House to rapidly confirm a replacement for Kennedy before the court's new term begins in October. Informally called the "war room," the group of Trump officials is expected to work closely with congressional Republicans in the coming weeks.

Shah, who serves as principal deputy press secretary under Sanders, "will oversee communications, strategy and messaging coordination with Capitol Hill allies," Sanders said in a statement, adding that Justin Clark, in his position as director of the Office of Public Liaison, will oversee outreach with key constituencies, coalitions, grass-roots organizations and allies.

The role of "sherpa" - a political heavyweight who will guide the upcoming nominee around Capitol Hill - has not yet been determined, according to one person involved in the process.

Most outside conservative groups, for the moment, are targeting Senate Democrats running for reelection in states won by Trump in 2016, but one veteran GOP strategist said Monday that advertising campaigns against moderate Republicans are not being ruled out by major donors.

And the map of Senate Democratic targets could soon expand. The Judicial Crisis Network, a conservative group that spent more than $10 million to support Gorsuch's confirmation, is considering pressure campaigns against Sens. Claire McCaskill, D-Mo., and Bill Nelson, D-Fla., according to two people familiar with the deliberations.

A spokesman for the nonprofit group said Monday that it would have a "seven-figure grass-roots program to inform voters in numerous red states." Last week, it began airing an ad in several states that showcased photos of Democratic leaders.